1. Field of the Invention
The present invention relates to a display technique and, in particular, to an electrophoretic display device which presents an image by moving charged particles in a fluid by means of a voltage applied between electrodes.
2. Description of the Related Art
With a rapid advance of digital technology, the mount of information handled by individuals has substantially increased. Along with this, thin-structured and power-saving display devices are now being developed. Particularly, development efforts have been made on liquid-crystal display devices as a display device that satisfy these requirements.
In the currently available liquid-crystal display devices, characters presented on a screen are sometimes difficult to view depending on a viewing angle with respect to the screen or due to light reflected from the screen. Flickering of a light source and low brightness on the screen cause eye fatigue. The current liquid-crystal display devices still have room for improvements in this regard. From the standpoint of power saving and comfort to the eyes, an electrophoretic display device draws attention as a thin-structured and power saving type display. An electrode structure in a matrix display device is disclosed in U.S. Pat. No. 4,655,897.
The electrophoretic display device disclosed in U.S. Pat. No. 5,053,763 employs a group of striped electrodes to present characters. By applying a negative voltage to all striped electrodes arranged on an anode electrode structure, an “erase” mode is activated. During a “hold” mode or a “write” mode, the striped electrodes are supplied with a positive voltage. For erasure, a negative voltage is applied.
It is pointed out that when the erase mode is carried out in the electrophoretic display device, portions of character lines on both sides of a character line blocked for erasure are also erased together. A partial erasure due to interference from an adjacent pixel takes place when a plurality of character lines are erased. The characters are difficult to read in such a case. Such a display is not acceptable.
U.S. Pat. No. 5,174,882 discloses a technique which avoids a partial erasure of an adjacent line by alternately arranging a striped electrode for a character line and an anode line in an electrophoretic display device.
In this technique, the added striped electrode is supplied with a voltage having a polarity opposite from that of the anode line for erasing one selected character or a group of selected characters during a selective erase mode. The disclosure states that a difference in polarity limits an erasure of a selected anode line to a particular character in the selected anode line, and that the erasure does not spread over a character which must not be erased.
There are two types of electrophoretic display devices. A first type is discussed in detail in U.S. Pat. No. 3,612,758.
Anode lines are arranged on one substrate and cathode lines are on the other substrate paired with the one substrate. Charged particles contained in a dispersing fluid interposed between the two substrates are placed close to or away from the substrate on the user's side to present an image. This type of electrophoretic display device is referred to as a vertical movement type electrophoretic display device in this specification.
A second type is a horizontal movement type electrophoretic display device in which anode lines and cathode lines are arranged on the same substrate, and charged particles suspended in a dispersing fluid as an electrophoretic medium are collected on an anode line or a cathode line to present an image. In the horizontal type electrophoretic display device as disclosed in U.S. Pat. No. 5,345,251, a conductive strip disposed on the same substrate is used to control the movement of charged particles which electrophoretically move between the anode line and the cathode line. The erasure of a portion of an adjacent character which remains displayed rather than being erased is controlled.
The electrode structure in a conventional electrophoretic display device disclosed in U.S. Pat. No. 5,345,251 is discussed below, and the disadvantages thereof are then discussed.
FIG. 14A is a sectional view of a conventional electrophoretic display device, and is a simplified version of FIG. 4 of the specification of U.S. Pat. No. 5,345,251. Charged particles 132 and a dispersing fluid 130 are held between two substrates 100 and 102. Anode lines 120, cathode lines 122, and guard electrodes 124 are arranged on the substrate 100. The guard electrode is arranged between a pixel A and a pixel B. Grid lines 110 are arranged on the substrate 102.
FIGS. 14B-1 and 14B-2 illustrate applied voltages, and the movement of charged particles during an erase mode and a write mode.
An erase operation is carried out with an anode line 120 supplied with zero V, a cathode line 122 supplied with +12 V, and a grid line 110 supplied with zero V. All charged particles, if negatively charged, are collected on the cathode line 122 as shown in FIG. 14B-1.
To maintain this state, the grid line 110 is supplied with zero V, the anode line 120 is supplied with +15 V, and the cathode line 122 is supplied with +12 V. FIG. 14B-2 shows applied voltages and the movement of charged particles in the write operation in which the charged particles are moved to the anode line 120 in the pixel A with the state in the pixel B held. The grid line 110 is supplied with zero V. In the pixel A, the anode line 120 is supplied with +15 V, and the cathode line 122 is supplied with zero V. In the pixel B, the anode line 120 is supplied with +15 V, and the cathode line 122 is supplied with +12 V.
There is no mention of the voltage applied to the guard electrode in U.S. Pat. No. 5,345,251. A method disclosed in U.S. Pat. No. 5,174,882 is here assumed. When a particular line is erased, the anode is supplied with zero V and the cathode is supplied with +12 V. The guard electrode is thus supplied with a voltage opposite in polarity from a selection scanning line (the cathode in this case), namely, −12 V. In an adjacent line, which is in a hold state, the anode is supplied with +15 V, and the cathode is supplied with +12 V. A voltage as high as 27 V is applied between the guard electrode and the anode, thereby causing a strong electric field spreading beyond the cathode. Negatively charged particles on the cathode move to the anode, and an image which must remain displayed is disturbed.
An adjacent pixel is also affected when an image is written on a selected pixel. According to U.S. Pat. No. 5,345,251, the anode line 120 is supplied with +15 V, and the cathode line 122 is supplied with zero V in a line to which a write operation is performed as shown in FIG. 14B-2. The grid line 110 on the counter substrate is supplied with zero V corresponding to a pixel to which a write operation is performed, and is supplied with a negative voltage corresponding to a pixel on which no write operation is performed. In an adjacent line in a hold state, the anode line 120 is supplied with +15 V, and the cathode line 122 is supplied with +12 V, and a voltage difference of 12 V is applied across adjacent cathode lines 122 with the guard electrode interposed therebetween. If the guard electrode 124 is supplied with a large negative voltage, negatively charged particles on the cathode line 122 move toward the anode line 120 as in a partial erasure, and an image to be held is disturbed. This movement is represented by an arrow M1 in FIG. 14B-2. If the guard electrode 124 is biased at zero V or a positive voltage to prevent this movement, the guard electrode 124 is unable to block the electric field taking place because of the voltage difference of 12 V between the adjacent cathode lines 122. Charged particles in the pixel A in a write operation move beyond the guard electrode toward the adjacent pixel B. No correct write operation is performed. This movement is represented by an arrow M2 in FIG. 14B-2.
Unintended movement of charged particles occurs not only during the selective erasure but also during a selective write operation. To avoid the unintended movement of the charged particles, the guard electrode must be biased at a proper voltage. The conventional art disclosed in U.S. Pat. Nos. 5,174,882 and 5,345,251 fail to provide a proper guideline of bias voltage setting.